JPS61153874A - Inversion preventing circuit - Google Patents

Abstract

PURPOSE:To prevent inversion from a white signal to a black signal by detecting the white signal component of a playback image signal, and amplifying and limiting only the white signal component while its level is higher than a set level and then imposing demodulation. CONSTITUTION:An HPF-I11 extracts only a luminance FM-modulated signal Y-FM from the playback signal and inputs it to an AGC circuit 12. The output signal A of the circuit 12 is inputted to an HPF-II13 and an HPF-III15. When the signal A does not contain any component corresponding to the white signal like a component passing through an HPF15, the output signal C of a level detector 16 is held at a low level and an HPF13 and a variable amplifier 14 perform normal operation. When the signal A contains a component corresponding to the white signal, on the other hand, the HPF13 switches a cutoff frequency to a high frequency with a high-level detection signal C and cut a low frequency component as a black signal component, thereby increasing the degree of amplification of an amplifier 14. The output signal D of the amplifier 14 is supplied to an FM demodulator 18 through a limiter 17 to prevent the inversion from the white signal to the black signal.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an inversion prevention circuit that prevents inversion of a video signal in, for example, a magnetic recording and reproducing device.

[Prior art and its problems] Conventionally, when recording a video signal on a video tape, a magnetic recording/playback device uses a 19127192 circuit to amplify the high frequency components before performing frequency modulation to reduce noise components. There is. In this case, when there is a sudden signal change, overshoot occurs in this part.

In particular, when the signal to be recorded changes significantly from black to white, the reproduced video signal of the normally modulated video signal drops to the black side, and the part that should be white on the playback screen becomes black, so-called inversion. A current position arises. In order to prevent such a reversal phenomenon, conventionally, as shown in Japanese Patent Publication No. 56-27927, a magnetic recording medium on which a frequency-modulated video signal is recorded is reproduced, and the frequency-modulated video signal is reproduced. a high-pass filter that filters and extracts high-frequency components of the reproduced video signal; a first vibration limiter that corrects level fluctuations present in the output of the high-pass filter; and the frequency-modulated reproduced video signal. or means for synthesizing an output signal from a low-pass filter that filters low-frequency components of the frequency-modulated reproduced video signal with the output of the first amplitude limiter, and a level of the output composite signal of the synthesizing means. An amplitude limiter comprising a second 16 II Ill limiter for eliminating fluctuations. ” is being considered. This amplitude limiter has features such as being able to very effectively remove the inversion phenomenon caused by sudden changes in the reproduced video signal, but the circuit is complicated, the cost is high, and it is small and expensive. There is a problem that it is not suitable for densification.

Furthermore, it is difficult to match the phase characteristics when synthesizing the high-frequency and low-frequency components.

[Object of the Invention] The present invention has been made in view of the above points, and provides an inversion prevention circuit that can simplify the circuit configuration and reduce costs, and that can reliably prevent the reversal phenomenon of the reproduced signal. The purpose is to

[Summary of the Invention] The present invention provides a white component detecting means for detecting a white component in a reproduced signal, and cuts a black component from the reproduced signal while a white component detection signal is output from the means. This amplifies only the white component and adds it to the limiter.

[Embodiment of the Invention] An embodiment of the present invention will be described below with reference to the drawings. 1st
The figure shows an example of implementation in a magnetic recording/playback device. In FIG. 1, reference numeral 11 denotes a high-pass filter, into which a reproduced signal from a video head (not shown) is input. The first high-pass filter 11 extracts only the luminance FM modulation signal Y-FM signal from the reproduced signal sent from the video head, and outputs it to the variable amplifier 14 via the AGC circuit 12 and the second high-pass filter 13. . Further, the output signal A of the AGC circuit 12 is sent to the third high-pass filter 15.

The third high-pass filter 15 extracts only the high-frequency component B corresponding to the white high-frequency signal from the reproduced FM signal and outputs it to the level detector 16 . This level detector 16
When the output signal level of the high-pass filter 15 exceeds a set value, this state is detected and a detection signal C is output to the second high-pass filter 13 and variable amplifier 14. The first second high-pass filter 13 is a filter configured to have a variable cutoff frequency using a variable reactance element or the like, and normally passes frequency signals in the entire range from black to white.
When the detection signal C is applied from the level detector 16, the cutoff frequency is switched to a higher one to cut off the low frequency component corresponding to the black signal and pass only the high frequency component corresponding to the white signal. The variable amplifier 14 also includes a level detector 16
The amplification degree is set to be high in the darkness where the detection signal C is given from
sent to. This limiter 17 is for removing the AM signal component in the input signal, and its output signal E is sent to the FM demodulator 18. The signal demodulated by this FM demodulator 18 is extracted as a reproduced luminance signal via a low-pass filter 19, and is processed by a color signal processing circuit (not shown).
The reproduced video signal is superimposed with the reproduced signal from the source and output as a reproduced video signal.

Next, the operation of the above embodiment will be explained with reference to the timing chart of FIG. The first high-frequency filter 11 generates a luminance FM modulated signal Y-F from the signal reproduced by the video head.
Only M is taken out and input to the AGC circuit 12. This AG
The C circuit 12 layer-widths the signal from the first high-pass filter 11 so that it always has a constant level, and transmits the output signal A to the second high-pass filter 11.
The signal is input to a high-pass filter 13 and a third^-pass filter 15. If the reproduced FM signal A output from the AGC circuit 12 does not contain a high-frequency component corresponding to a white signal that passes through the third high-pass filter 15, the output signal of the level detector 16 C is held at low level. When the output signal C of the level detector 16 is held at the 0 level, the second high-pass filter 13 and the variable amplifier 14 are operating normally.

Therefore, from the output signal A from the AGC circuit 12, a signal in the normal frequency range from black to white is extracted by the second high-pass filter 13 and amplified by the variable amplifier 14. The signal output from the variable amplifier 14 is limited to a predetermined level by the limiter 17, and then the FM demodulator 1
8. The signal demodulated by the FM demodulator 18 is extracted as a reproduced luminance signal by a low-pass filter 19.

If the signal A output from the AGC circuit 12 contains a white high frequency component that may cause signal inversion, the high frequency component is filtered by the third high pass filter 15 as shown in FIG. The signal B is output as a signal B and input to the level detector 16. This level detector 16
When the level of the output signal @8 of the ^-band filter 15 exceeds the set level, this state is detected and a high-level detection signal C with a slope at the front and rear is output as shown in FIG. It is input to a band pass filter 13 and a variable amplifier 14. The slope before and after the high level portion of the detection signal C is the second high-pass filter 13 and the variable J! This is to prevent discontinuity from occurring in the FM wave that is the output of the fIIA device 14. When the second high-frequency filter 13 is given the detection signal C from the level detector 16, it switches the cutoff frequency to a higher side, cuts the low frequency component that is the black frequency component, and cuts the low frequency component that is the black frequency component. Only certain high frequency components are extracted and output to the variable amplifier 14.

When the variable amplifier 14 receives the detection signal C from the level detector 16, the amplification degree becomes high, and the white frequency signal output from the second high-pass filter 13 is amplified to a sufficiently large signal. The signal outputted from the variable amplifier 14 is cut at a predetermined level by a limiter 17 to become a signal E. The signal E output from the limiter 17 is demodulated by the FM demodulator 18, and then taken out as a reproduced luminance signal via the low-pass filter 19. Therefore, while the detection signal C is being output from the level detector 16 as described above, the black component is cut and only the white component is amplified and input to the limiter 17. This prevents signal reversal from white to black.

Further, after that, when the detection signal C for the white component is no longer output from the level detector 16, the second high fall filter 13
and the variable amplifier 14 returns to the above-mentioned initial setting state,
An operation is performed according to the setting state.

[Effects of the Invention] As detailed above, according to the present invention, a white component detection means for detecting a white signal component in a reproduced signal is provided, and while a white component detection signal is output from this means, the above-mentioned Since the black component is cut from the reproduced signal and only the white component is amplified and added to the limiter, it is possible to reliably prevent the inversion of the white signal to the black signal with a simple circuit configuration, making it smaller and more dense. Therefore, it is possible to provide an inversion prevention circuit that is suitable for use and can reduce costs.

[Brief explanation of the drawing]

The drawings show one embodiment of the present invention, so FIG. 1 is a block diagram showing the circuit configuration, and FIG. 2 is a timing chart for explaining the operation. 11... First high-pass filter, 12... AGC circuit,
13... 2nd^ band filter, 14... variable amplifier,
15... Third high-pass filter, 16... Level detector, 17... Limiter, 18... FM demodulator, 19...
...Low pass filter.

Claims (1)

[Claims] A first high-pass filter filters and extracts the high frequency component of the frequency-modulated reproduced video signal, and a white signal component in the output signal of this first high-pass filter is detected, and the white signal component is equal to or higher than a set level. While the white component detecting means outputs a detection signal, the output signal of the first high-pass filter is always taken out as it is, and when the white component detecting means outputs a detection signal, the first high-pass filter A second high-pass filter extracts only a white component signal from the output signal of the pass filter, and while the output signal of the second high-pass filter is input and the white component is detected by the white component detecting means, The present invention is characterized by comprising a variable amplifier whose amplification degree increases, a limiter which limits the output signal of the variable amplifier to a predetermined level, and an FM demodulator which demodulates the output signal of the limiter to obtain a reproduced luminance signal. Reversal prevention circuit.